A number of non-instrumented devices have heretofore been developed for measuring analytes in fluid samples, such as dipsticks, reagent-impregnated slides, and the like. However, although these devices have the advantage of being inexpensive and easy to use, they have the disadvantage of not providing a means for determining if sufficient sample has been introduced to the device to provide an accurate measurement of the analyte. For example, if an insufficient amount of sample is introduced to the device, a false low reading may be obtained.
Medical science in particular has an increasing need for quick, accurate determination of analytes in blood or other body fluids. Traditionally, assays for analytes have been performed by laboratories and have required skilled technicians, complex apparatus and reagents, and considerable time in order to determine accurate results. Numerous qualitative and some quantitative devices and methods have been developed which allow the lay person to perform self-testing at home or outside of a traditional laboratory. Many of these devices and methods include test strips or dip sticks which can be exposed to blood or other body fluids in order to identify or quantify a component of body fluid. A common example of this technology includes the various test products for determining the concentration of blood glucose in diabetics.
The determination of the concentration of glucose and other analytes has heretofore been performed by various devices and methods using either urine or blood as the body fluid sample. The most common of these test are dip sticks from testing the concentration of glucose in urine. The dip sticks are dipped into a sample of urine and then undergo a color change. The color of the dip stick is compared to a chart of color references on the label of the container providing the dip stick. When the color of the dip stick is matched to the color reference, one determines the approximate glucose concentration from the color reference. Similar tests exist wherein paper strips are used to determine the concentration of glucose in whole blood. These tests are also conducted by comparing the amount of color formation of the paper strip to a standard. These semi-quantitative tests do not accurately determine the concentration of an analyte in the blood being tested unless an instrument is also used.
Devices have been proposed which detect and quantitatively measure analytes in body fluids, such as whole blood. When a drop of blood or other body fluid, such as urine, is applied to the device, the sample is drawn into consecutive compartments which separate solids from the liquid. In the case of whole blood, red blood cells are separated from the plasma. The filtered sample is then contacted with an enzymatic reagent to produce hydrogen peroxide. The hydrogen peroxide is reacted with a dye in a linear measurement zone, and the sample is drawn into a zone which serves to meter the quantity of sample which enters the measurement zone.
Although this type of device requires merely one or two drops of sample (about 40 to 80 .mu.liters), if a sufficient amount of sample is not applied to the device a falsely low reading will be obtained. If, for example, the user does not wait for a full hanging drop to form from a finger stick, and wipes only a small quantity of blood on the entry to the device, less than 40 .mu.liters of sample may be applied to the test device. If a sample of less than 40 .mu.liters is applied to the device, this amount may conceivably be enough to initiate the reaction and cause a color change in the measurement zone, but not enough to fill the zone to full capacity, thereby giving a falsely low reading. One possible precaution against taking a false reading because of insufficient sample would be to have an end-of-test indicator in the draw zone, which only changes color when the draw zone is filled to capacity. However, an end-of-test indicator only signals that it is all right to read the results because a sufficient amount of sample has been drawn into the device to complete the test. If an insufficient amount of sample has been added, the test is ruined, because the chemical reactions in the device would have been initiated, but there would not be sufficient sample to provide an accurate result. Worse still, the user could ignore the absence of the end-of-test indicator and read the test result, which would be falsely low due to insufficient sample. Another problem with adding insufficient sample could be if the user adds more blood several minutes after addition of the first blood drop in an effort to complete the test. This could also give erroneous results due to discontinuous flow of the sample through the measurement zone (which could cause flow problems and time interval dependent changes in chemical reactions). Clearly, the optimal situation would be to delay the start of the test until a minimum amount of blood required to begin and complete the test has been transferred to the device.
Another problem connected with strip assay devices is that, as the fluid flows through the detection zone, the fluid flow is not even. Because the fluid flows more rapidly in the center of the zone, a "rocket-shaped" colored zone is formed. It may not be possible to determine the true end of the colored column.
Wright, in U.S. Pat. No. 3,915,647, discloses a device for determining the concentration of a substance in a fluid comprising a fluid receiving cavity of predetermined volume with an egress of relatively small dimensions. The fluid is disposed in the cavity and the proper amount is present when the cavity is totally filled or filled to a mark. The reagents which provide the colorimetric determination are preferably disposed in the cavity prior to the addition of the fluid. However, there is no provision for delaying the start of the reaction until sufficient fluid is added to the cavity.
Allen et al., in U.S. Pat. No. 4,987,085, disclose a filtering metering device in which various metering systems may be used to insure the substantial reproducibility of the amount of fluid sample absorbed by the reactant pad. The systems may involve absorbent pads separated by a substantially non-wettable mesh or a film which serves to wipe away excess sample from the reactant pad. There is no indication that the assay will not be started until sufficient sample is applied to the pad; rather, this device is concerned with excess sample applied to the device.
Allen et al., in U.S. Pat. No. 4,999,287, disclose a stripstick for analysis which includes means for automatically metering the volume of a sample so as to prevent excess of sample from interfering with the assay. Where the sample pad is to serve as the sample volume measuring device, the pad will have one side exposed for receiving the sample and the other side in contact with a porous, non-wettable film which is in contact with an absorbent layer. The sample will saturate the sample pad, and any residual fluid will overflow through the porous film and be absorbed by the absorbent layer so that a fixed amount of sample fluid is taken up by the pad. There is no provision for ensuring that a minimum amount of sample is applied to the pad.
Grenner, in U.S. Pat. No. 4,906,439, discloses a biological diagnostic device comprising a diagnostic test element and a sample application unit comprising a fluid delivery element comprising a layer having a plurality of grooves or channels in the surface thereof which is adjacent to the test element. The grooves can be made very small so as to deliver a small volume of precisely metered sample fluid to the device.
Engelmann, in U.S. Pat. No. 4,738,823, discloses a test strip with a preselected sample absorption capacity. Absorbent material is provided to remove excess sample applied to the reagent strip. However, there is no provision for ensuring that enough sample has been applied to conduct a test.
Burkhardt et al., in U.S. Pat. No. 4,810,470, disclose a diagnostic device comprising a first bibulous matrix that is adjacent to and in contact with a second bibulous matrix. The second bibulous matrix has been treated with a reagent suitable for detecting a specific analyte. In addition, the reagent-treated second bibulous matrix and a portion of the untreated first matrix are covered with a liquid-impermeable coating or film which serves to assist in metering the liquid sample into the first and second bibulous matrices and to act as a barrier to prohibit the test sample from directly contacting the reagent-treated bibulous matrix. The reagent-treated assay area of the matrices absorbs liquid test sample only up to the point of matrix saturation. This device prohibits excess sample from entering the assay area of the device, but does not ensure that sufficient sample enters the assay area of the device.
Lange et al., in U.S. Pat. No. 4,605,629, disclose a method for improving elution of reagent from a reagent strip. The reagent strip is provided with a handle, on the lower part of which is affixed an absorbent carrier impregnated with a reagent. The absorbent carrier is pressed against the handle by a thin, carrier-covering mesh which is stuck or sealed on to the handle on opposite sides of the carrier.
Zuk et al., in U.S. Pat. No. 4,435,504, disclose an immunochromatographic assay with a support having bound "mip" or antibody and a second enzyme. This assay measures the amount of analyte in a sample solution of a body fluid by combining a premeasured volume of sample with a premeasured volume of a solution of enzyme labelled analyte and immunochromatographing the solution or employing a combination of enzymes wherein one enzyme is the label and the other enzyme is affixed to the chromatographic support. The assay of this invention is performed by contacting the immunochromatograph with the sample containing solution. The sample traverses a region of the immunochromatograph by elution or solvent transport. The device used in this assay has a region in which the antibody is non-diffusively bound to a bibulous support. The analyte from the sample and its enzyme labelled conjugate traverse this zone along with the solvent. The analyte and its enzyme labelled analogue become bound to the support through the formation of an antibody complex. The signal producing system provides the area in this region with a color change which identifies the distance from a predetermined point over which the analyte and its enzyme labelled conjugate have travelled. In this manner, a quantitative determination of the analyte can be made. This assay does not directly test whole blood, and requires accurate volumetric measurement of the sample and the enzyme conjugate solution and dilution of the sample by a separately applied solvent. Furthermore, using this method to determine analyte concentration requires the use of a separate signal producing system. There is no immediate determination of the concentration of an analyte.
Zuk, in U.S. Pat. No. 4,594,327, discloses an assay method for whole blood samples. This assay requires at least one specific binding pair which is substantially uniformly bound to a solid bibulous element. The method of this invention requires that the sample be mixed in an aqueous medium with a binding agent, as well as a separate signal producing system such as discussed above. There is no provision of a self-contained unit that accurately determines the concentration of an analyte without the use of additional solvents or reagents.
Sloan et al., in Clin. Chem. 30:(10) 1705-1707 (1984), disclose a test strip which provides a quantitative measurement of chloride and sodium concentrations in urine. The test strips rely on wicking alone, and do not provide an additional capillary channel to speed up movement of the sample. The porous matrix typically requires between 15 and 30 minutes to draw urine up the entire measurement zone. This device does not provide a rapid quantitative test, a channel or a separation means for solids.
Hochstrasser, in U.S. Pat. Nos. 3,964,871 and 4,042,329, discloses a method and device for detecting either glucose or cholesterol. The device is dipped into a sample of body fluid, and the fluid reacts with an analyte. The concentration of the analyte correlates with a color intensity scale which translates into an approximate quantitative determination of the analyte. These tests, however, do not analyze whole blood.
Blatt et al., U.S. Pat. No. 4,761,381, disclose a volume metering capillary gap device for applying a liquid sample onto a reactive surface. The device controls a liquid volume flowing onto a reactive surface by means of an overflow chamber. The capillary channel leading to the overflow chamber is controlled so that liquid cannot flow back into a reaction chamber. The analytical method introduces liquid very quickly into the device (within 2 seconds) in order to prevent slow entry by simultaneous capillary action in the channel and wicking through the porous matrix at the bottom of the device. The geometry of the detection chamber determines the volume used for the test. Two compartments are connected in parallel to the sample entry port, i.e., liquid flowing from the entry port into the overflow chamber does not flow through the reaction chamber. The geometry of the reaction chamber, through rectangular, is not channelled, nor is it suitable for a measurement scale. Although this device can receive blood as a sample fluid, there is no means for separating cells from plasma.
Ramel et al., U.S. Pat. No. 4,959,324, disclose a self-contained assay device using two strips separated by a gap wherein a flow path is completed by movement of a sample receiving pad into the gap. The movement of the sample receiving pad also results in release of a reagent solution which is then transported through the pad into a quantitation area, where the amount of analyte may be determined.
Hillman et al., U.S. Pat. No. 4,756,884, disclose a capillary flow device which provides for measuring a sample, mixing the sample with reagents, defining a flow path and reading the result. The capillary tube of this device provides the sole driving source for the movement of liquid through the device. The use of this device primarily involves tests requiring blood agglutination and optical readers to determine test results. There is no self-contained quantitative analysis means for measuring analytes.
Vogel et al., U.S. Pat. No. 4,477,575, disclose a process and composition for separating plasma and serum from whole blood. The device uses glass fibers having an average diameter of from 0.2 .mu. to 0.5 .mu. and a density of 0.1 g/cm.sup.2 to 0.5 g/cm.sup.2. The total volume of the plasma or serum separated from the blood is limited to at most 50% of the absorption volume of the glass fiber layer. Other fibers may be useful in forming the matrix with the glass fibers. There is no provision for metering plasma flow through the device nor a quantitative analysis of an analyte.
Ramel et al., U.S. Pat. No. 4,959,324, disclose an assay device for detecting or quantifying an analyte by measuring the distance of a detectable signal from a predetermined site. The device uses two strips separated by a gap, where a flow path is completed by movement of a sample receiving pad into the gap. The movement of the sample receiving pad also results in release of a reagent solution which is then transported through the pad into the quantitation area where the amount of analyte may be determined.